A. Field of Invention
The present invention relates generally to methods and devices for providing resistance to flow of fluid through a flow passageway and more particularly to a method for achieving high levels of fluid resistance at a low cost and to devices for practicing said method.
B. Description of Related Art
In hydraulic and in fluidics systems generally, it is frequently desirable to provide a means for producing a high level of resistance to flow through a passageway. Commonly used methods for providing resistance to the flow of a fluid, in their simplest form, frequently achieve desired levels of resistance to flow by simply reducing the internal diameter of an orifice through which the fluid must pass. While this method is simple, it is limited by the fact that it has been found to be difficult to produce an orifice smaller than 0.005 inches in diameter in an economical manner. Conventional, relatively low cost methods of molding and machining are currently notable to form orifices smaller than 0.005 inches in diameter. For this reason, methods of achieving high levels of restriction to flow using porous metals formed by sintering powdered metals have been tried. Such methods comprise a step of simply pressing a cylinder or plug of porous, sintered material into a passageway so as to partially block the flow passage, with enough compressive force on the plug to frictionally retain the plug of the porous, sintered material within the flow passage walls so that flow proceeds axially through the sintered material. With such methods the amount of resistance that is provided is dependent upon the number and size of the pores or open spaces in the densest cross section of the material, and to some extent, on the length of the resistor and the smallness of the interstitial spaces. To some extent, the convolutions of the expected flow paths through the material may contribute to providing some resistance to flow, but reducing the cross sectional area of the flow passage, which occurs at the section of maximum density results in the majority of resistance in most cases. With the use of materials of uniform porosity, the effective resistance is expected to occur at whichever end surface is exposed to higher pressure. The amount of resistance achieved by such methods is limited by the size of the flow passage and any strength requirements imposed by the particular application, as well as the density of the material used. The retentive force that can be achieved by the interference of the plug within the passageway is limited by the need to avoid compacting the material by imposing enough force on the sintered material to alter the characteristics of the material, for example, by increasing the density of the material and reducing the interstitial space within the material, thereby reducing the porosity of the material and changing the resistance. In addition, a plug of sintered metal used as a fluid resistor will act as a fine pore filter, a characteristic that may be desirable in some circumstances but which causes the simple plug method to be susceptible to failure by the clogging of the pores. As the sintered material becomes clogged, the resistance presented by the plug changes and may exceed the tolerances of the system. In such methods, both the filtering and resistance functions are performed by the transverse surface at the upstream end because the cross-section presenting the least open area is normally the upstream, transverse end surface. Therefore, the porosity of the upstream end surface effectively determines the resistance achieved, and the rate of clogging of the resistor depends upon the number and size of the pores in the upstream, transverse end surface. For this reason, the resistance achieved by such methods is expected to change constantly at rates that depend upon the size and density of the sintered metal material which may change upon installation. In such plugs, the pressure drop across the resistor occurs over the entire plug when it is initially installed but as the upstream surface becomes clogged that surface comes to provide most or all of the effective resistance and the pressure drop provided by the resistor may be expected to occur across the surface of the upstream end. Due to the foregoing characteristics, the usefulness of the method of providing fluid resistance by means of the simple insertion of a sintered metal plug within a passageway is substantially limited except in applications where the functions of the resistor are simply to provide an uncalibrated bleeding of fluid. Such methods are not useful in application where clogging may be a problem or where is necessary to accurately control the resistance provided or provide a constant level of resistance to flow. Further, the retention of the sintered material by friction imposed by radial compression is limited by the need to refrain from altering the porosity of the material.